The present invention relates to a double heterostructure light-emitting semiconductor device of the surface light-emitting type having double heterojunctions.
A light-emitting semiconductor device having a double heterostructure including GaAlAs layers or a GaAlAs layer and a GaAs layer is known. For example, a light-emitting semiconductor device having a structure as shown in FIG. 1 is disclosed in "IEEE Transactions On Components Hybrid and Manufacturing Technology; Vol. CHMT-3, No. 4, December 1980." In FIG. 1, a first electrode 7 is formed on one surface of a p-type GaAs substrate 1, and a current confinement layer 2 made of an n-type GaAs is formed on the other surface. A confined current conduction layer 9 is formed in a selected region of this current confinement layer 2. A light-emitting layer structure 10 is formed on the current confinement layer 2 including the current conduction layer 9. This structure 10 has a p-GaAlAs cladding layer 3, a p-GaAlAs active layer 4, and an n-GaAlAs cladding layer 5 sequentially superposed on the layer 2 and includes double heterojunctions. A light exit layer 11 made of a silicon nitride film is formed in a region within a capping layer 6 of an n.sup.+ -GaAs ohmically contacting the surface of the n-GaAlAs cladding layer 5. The existing layer 11 is opposed to the current conduction layer 11. Since this layer 11 has a refractive index smaller than that of the GaAlAs layer, a light generated in the light-emitting layer structure 10 can be efficiently transmitted outside through the light exit window 12. A second electrode 8 is provided on the surface of the capping layer 6 excluding the region where the layer 11 exists. Positive and negative terminals of a DC voltage source are respectively connected to the first and second electrodes 7 and 8.
The light-emitting semiconductor device is desired to produce light beams having a high brightness and a spot-shaped cross section. In order to generate very bright light beams, it is necessary to sufficiently reduce the diameter of the confined current conduction layer 9 as compared with that of the window 12, i.e., to sufficiently confine the current path through which a current between the electrodes 7 and 8 flows. Further, when a predetermined DC voltage is applied between the electrodes 7 and 8, it is desirable that a series resistance to the current flowing across the structure 10 is as small as possible. Moreover, to obtain sectional spot-shaped light beams, it is desired to concentrate the current paths between the electrodes 7 and 8 on a predetermined region. When considering the device shown in FIG. 1 from the above-described standpoint, it is clear that the current passed through the conduction layer 9 obliquely moves the structure 10 toward the layer 6. Therefore, the series resistance of the structure 10 for the oblique current flow increases, and the light emitting efficiency is lowered when a constant voltage is applied between the electrodes 7 and 8. In addition, since a current does not substantially flow from the layer 9 toward the layer 11 of the silicon nitride film, the light beams generated from the light exit window present a ring-shaped near field pattern.